-LRB- CNN -RRB- -- Inspectors overseeing the destruction of Syria 's chemical weapons have visited 21 of the 23 sites they want to inspect by a Friday deadline , but security concerns have prevented visits to the remaining two , the world 's chemical weapons watchdog said Monday .

`` Efforts ... to ensure the conditions necessary for safe access to those sites will continue , '' the Organisation for the Prohibition of Chemical Weapons said in a statement released Monday .

OPCW spokesman Michael Luhan added that the two remaining sites are in areas contested by government forces and opposition militants -- part of a conflict that has raged in the country since 2011 .

A joint OPCW-United Nations team is overseeing Syria 's efforts to destroy its chemical weapons , which the country agreed to do earlier this year under international pressure . The U.N. Security Council resolution gives Syria until mid-2014 to destroy that arsenal , and charges inspectors with completing their initial inspections of all Syrian chemical weapons and storage facilities by November 1 .

But Syria 's civil war presents inspectors with significant challenges , including having to cross front lines and move through areas controlled by rebels . The OPCW previously has said that cooperation by rebels , in the form of temporary cease-fires to allow inspectors safe passage , would be key if deadlines are to be met .

Luhan said Monday that the United Nations is negotiating for the inspectors ' safe passage to the two remaining sites . The inspectors will be deployed as soon as their safety is guaranteed , he said .

Monday 's statement by the OPCW came a day after the group said Syria met its Sunday deadline for submitting a plan to get rid of the chemical weapons . Damascus disclosed the 23 sites earlier this year .

On August 21 a chemical attack outside Damascus led the United States and its allies to call for military intervention in Syria 's civil war -- a situation that was defused in mid-September , when Damascus agreed to a U.S.-Russian plan to give up its chemical weapons stockpile .

But the Syrian opposition has protested that the Syrian government has done nothing to stop the killing of civilians using conventional weapons .

According to the United Nations , more than 100,000 people have died in the Syrian conflict . It began in March 2011 when government forces cracked down on peaceful protesters during the Arab Spring movement and has morphed into a full-blown civil war .

A proposed conference in Geneva , Switzerland , between Syrian government officials and opposition leaders , intended to broker an end to the war , has been delayed several times , but is tentatively scheduled for the end of November .

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Inspectors were given until Friday to visit 23 chemical weapons sites

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Team has yet to reach two sites because of security concerns , OPCW says

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Team 's task to oversee destruction of Syria 's chemical weapons comes amid civil war

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U.N. giving Syria until mid-2014 to destroy its chemical weapons arsenal